1. Field of the Invention
The present invention relates to a color image forming apparatus.
2. Related Background Art
In conventional monochromatic image forming apparatuses, the input image signal received from a host apparatus is subjected for example to a tonal conversion to obtain an image signal specific to the apparatus, and the image formation is carried out, based on said signal. The input image signal is proportional, in general, to the luminocity or the density. On the other hand, color image forming apparatuses can be divided into two types. In the first type, plural color component signals received from the host apparatus are respectively subjected to tonal conversions to obtain color component image signals specific to the apparatus, and said signals are utilized in forming color component images. This type requires input color component signals corresponding to the color components specific to the apparatus. In the second type, plural color component input signals received from the host apparatus are subjected to masking, tonal conversion etc. to obtain plural color component signals specific to the apparatus, which are utilized in forming color component images. In this type, the input color components may be different from the color components utilized in image formation. For example R, G, B input components can be utilized to separate color component signals of C, M, Y and K (black) which are utilized for image formation.
In the above-mentioned monochromic image forming apparatus, the host apparatus can form an image according to its intention, since the input image signal and the signal used in image formation have almost one-to-one correspondence.
On the other hand, in the color image forming apparatus of the first type explained above, the host apparatus has to supply color component signals specific to the image forming apparatus, and is unable to form an image matching its intention, unless characteristics specific to the image forming apparatus, for example the positions of the color components in the color space, are made known to the host apparatus. Also said input color component signals have to be changed according to the type of the image forming apparatus, since they are specific to the image forming apparatus.
The color image forming apparatus of the second type is not associated with this drawback, as it can receive standard color component signals such as RGB luminance signals in the NTSC format, and convert said signal into color component signals specific to the apparatus, such as C, M, Y, Bk density signals. However, in the apparatus of this type, if given an ability of black image formation by extracting black component, the input image signals are subjected to masking so that other color components are inevitably mixed in, even when image formation with black color only is desired. Also if the input image contains a character area such as black characters, satisfactory character image cannot be obtained unless the input image signals corresponding to the characters are adjusted by the host apparatus to a value suitable for the characters. Therefore, in the apparatus of this type, an image intended by the host apparatus cannot be obtained only through the control of the input image signals to the image forming apparatus.